1. Field of the Invention
The present invention relates to an automatic focus adjusting device, and more particularly to an active type automatic focus adjusting device in which infrared light is projected onto an object and the light reflected by the object is sensed by a differential type senser to carry out automatic focus adjustment.
2. Description of the Prior Art
Below, various prior art automatic focus adjusting devices of this kind (hereinafter called "AF device") will be explained with reference to FIGS. 1 to 7.
FIG. 1 shows, in general, a construction of an example of the AF device.
In association with the movement of a lens group 1 taking part in an focusing operation in a photographic lens system, a light projecting element 3, consisting of a laser diode, a near-infrared ray emitting diode and so on, projects a light spot onto an object 5 through a light projecting lens 4. The light reflected by the object 5 is received by a light sensing element 8, having two divided areas 8A and 8B consisting of PIN photodiodes, charge-coupled devices and so on, through a light receiving lens 6 and a visible light cut filter 7. The light sensing element 8 also moves in association with the movement of the lens group 1. A distance to the object 5 is detected by a control circuit (hereinafter called "AF circuit") 10 by means of an output of the light sensing element 8 so as to control the driving of a focusing lens group driving motor 9 and set the lens group 1 at an in-focus position, so that an image of the object 5 is formed sharply on an image forming plane 2 of an image pickup tube (or an image forming plane of an image pickup element, or film, in another type camera) by the photographic lens system including the lens group 1.
In the light sensing element 8 in the case of this AF device, the area 8A is arranged at the side near the light projecting element 3, and the area 8B is arranged at the side distant from the element 3.
In a distance measuring operation of the AF device shown in FIG. 1, assuming that when the object 5 has a distance l2 from the image forming plane 2, as is shown in FIG. 2(a), reflected light of a light spot image S is received by the light sensing element 8, with the same light quantity on the two areas 8A and 8B. In this case, in the light sensing element 8, the difference VA-VB between an integration value VA, which is obtained by integrating an output of the area 8A with reference to time, and an integration value VB, which is obtained by integrating an output of the area 8B with reference to time, is nearly zero. As to an optical path, light projected by the light projecting element 3 reaches the object 5 via an optical path b1, and the light reflected by the object 5 forms an image on the light sensing element 8 via an optical path b2. Now, suppose that the lens group 1 is at an in-focus position and the object 5 moves forward to have a distance l1. Then, the in-focus position of the lens group 1 shifts backward and a far-focus condition is assumed. On the other hand, if the light projecting element 3 and the light sensing element 8 are positioned as they stand, projected light reaches the object 5 through the optical path b1, and the light diffused and reflected by the object 5 forms an image on the light sensing element 8 through an optical path b'2, so that, as shown in FIG. 2(b), the position of the formed light spot image S is largely deviated to the side of the area 8B and the difference VA-VB does not equal zero. According to the sign of VA-VB, the AF circuit 10 drives the motor 9 to rotate in the normal or reverse direction based on the sign of the difference VA-VB so as to advance or withdraw the lens group 1. When, as mentioned above, the difference VA-VB is negative, the lens group 1 is advanced.
Now, suppose that when, in FIG. 1, the lens group 1 has been advanced to the position 1', an image of the object 5 having the distance l1 is formed sharply on the image forming plane 2. In association with the lens group 1, the light projecting element 3 and the light sensing element 8, move to the positions 3' and 8' respectively, by means of cams and so on (not shown). Then, a projected light path becomes a1 and a refrected light path becomes a2. As a result, as is shown in FIG. 2(a), the light spot image S moves to the median position between the areas 8A and 8B on the light sensing element 8. At this time, the difference VA-VB becomes almost zero, and the motor 9 stops. On the other hand, when the object 5 moves to have a distance l3, the lens group 1 moves in the reverse direction so as to carry out the focusing operation until the difference VA-VB becomes zero. The projected light path and the reflected light path in this case are shown with c1 and c2.
FIGS. 3 to 6 respectively show examples of the conventional AF devices carrying out the distance measuring on the same principle as that of the device shown in FIG. 1. The AF devices differ in the form of a light projecting system and a light sensing system from the device shown in FIG. 1. Hereby, members having the same reference numerals as those in FIG. 1 are the same members.
In the device shown in FIG. 3, the light projecting element 3 projects a light spot onto the object 5 through the focusing lens group 1 via the light projecting lens 4 and a prism 11 having a reflecting plane 11a composed of a cold mirror arranged in a photographic light path, and the light reflected by the object 5 is received by the light sensing element 8 arranged outside of the camera through the light receiving lens 6 and the visible light cut filter 7. Namely, the device is of the so-called half-TTL distance measuring type, in which the light projecting element 3 and the image forming plane 2 are arranged in the optically conjugate positions with each other. The focusing lens group 1 is moved by the motor 9 which is controlled by the AF circuit 10, mechanically in association with the light sensing element 8.
In the device shown in FIG. 4, the light projecting element 3 projects a light spot onto the object 5 through the focusing lens group 1 via the light projecting lens 4 and the reflecting plane 11a of the prism 11 arranged in the photographic light path, and the light reflected by the object 5 is received by the light sensing element 8 through the light reflecting lens 6 and the visible light cut filter 7 via the focusing lens group 1 and a reflecting plane 11b of the prism 11. Namely, the device is of the so-called TTL distance measuring type, in which the light projecting element 3 and the light sensing element 8 are arranged in optically conjugate positions with the image forming plane 2, and a projected light beam and a sensed light beam pass at the positions distant from each other near the periphery of the pupil of the lens group 1.
Hereby, the light projecting element 3 and the light sensing element 8 are fixed, and, therefore, the mechanical association thereof with the photographic lens is not required.
The device shown in FIG. 5 is a modification of the device shown in FIG. 4, in which the reflecting plane 11a of the prism 11 is formed such that a projected light beam coincides with the photographic optical axis.
In the AF device shown in FIG. 6, the same light projecting system as that shown in FIG. 4 is used, and an image pickup element 14 is used as a light sensing element operative for both adjusting the focus and picking up the image. An image signal produced by the image pickup element 14 is divided by a distributing circuit 12 to be supplied to the AF circuit 10 and an image pickup circuit 13.
FIG. 7 shows a light sensitive plane of the image pickup element 14 of the device shown in FIG. 6. When it is used for focus detection, signals from two zones 14A and 14B are supplied to the AF circuit 10 by the distributing circuit 12. Therefore, it is necessary that, during distance measurement, an infrared ray should reach the image pickup element 14 and that, during image pickup operation, the infrared ray should be excluded.
In the preceding conventional examples, the device shown in FIG. 1 has a merit such that because the light projecting lens 4 and the light receiving lens 6 are arranged outside of the photographic lens group 1, it is possible to make the light projecting lens 4 and the light receiving lens 6 large so that light can reach longer distance. However, it is inconvenient that the device, on the whole , cannot be made compact. On the other hand, the device shown in FIG. 4 has the merits and demerits reverse to that shown in FIG. 1. Furthermore, the highly precise mechanical association of the photographic lens group 1 with the light projecting and receiving systems is not required, so that the construction can advantageously be simplified.
The device shown in FIG. 3 has the intermediate character between that shown in FIG. 1 and that shown in FIG. 4.
In the case of the device shown in FIG. 5 in comparison with the type shown in FIG. 4, the length of the base line of the light projecting and receiving systems is short. It is not convenient in obtaining the distance measuring accuracy, and, however, like the device shown in FIG. 3, a projected light beam is conveniently situated at the center of the view finder even in the out-of-focus condition. Hereby, in the case of all of the preceding devices, the image of the light spot formed on the object 5 by the light projecting element 3 is formed on the optical axis of the photographic lens in the in-focus condition. Namely, the distance measuring zone of all the preceding devices is at the center of the view finder, so that the AF device free from parallax, can be realized.
Furthermore, in the case of the device shown in FIG. 6, a light sensing area of the image pickup element 14 operating as an AF light sensing element is equal to a light receiving area of the photographic lens, so that, as compared with other types of the devices, a larger light sensing area can be obtained. It is advantageous from the point of view of the light reaching distance.
Furthermore, in the case of the device shown in FIG. 6, a signal produced by the image pickup element 14 is divided to be supplied to the AF circuit 10 and the image pickup circuit 13. In practice, this division is carried out time-divisionally, so that the device of this type is suited for a system such as a still video camera in which distance measurement is completed prior to exposure. However, since the conventional devices are constructed as mentioned above, an object whose reflection factor in the infrared range is even can be focused correctly, while an object which has different reflection factors in the infrared range is inconveniently accompanied with distance measurement error (hereinafter called "contrast blur").
Below, the contrast blur will be explained with reference to FIGS. 8(a)-8(g) and 9(a) and 9(b).
In FIG. 8(a), S is an infrared ray spot image on the light sensing element 8. The radius of the spot image S is assumed 1. A hatched portion at the left-hand side of the spot image S is a part reflected by an object whose infrared ray reflection factor is k1, while the right-hand side portion is a part reflected by an object whose infrared ray reflection factor is k2. Now, define k=k2/k1, and k assuming the value 1-.infin.. Abscissa l in FIGS. 8(a) and 8(b) shows the position of the boundary between the parts having different reflection factors, with the geometrical center O of the spots image S assumed as the origin. In the state shown in FIGS. 8(a), G is the center of the signal intensity of the spot image S. A signal in the left-hand side portion of the center G is balanced with a signal in the right-hand side portion. FIG. 8(b) shows the relation of the distance E (OG) to the position of the boundary of the reflection factor in the abscissa, in the case of k= 8. Namely, when the position l of the boundary of the reflection factor is about 0.6, OG, namely E, assumes the maximum value of about 0.7. It goes without saying that when l=-1 or l=1, the spot is being projected onto an object having even reflection factor, and E=0.
Below, a graph representing the distance E of FIG. 8(b) will be explained.
In FIG. 8(c), a circle 101 represents an infrared spot image of an only white object whose reflection factor is high. O represents the geometrical (optical) center of the spot image 101. The abscissa l represents the position of the boundary of the parts having different reflection factors.
FIG. 8(c) shows spot light from an object having even reflection factor, so that the intensity center G1 of the reflection signal coincides with the optical center O. Accordingly, the distance E1 between the reflection signal intensity center G1 and the optical center O is zero.
FIG. 8(d) shows a case where the reflection factor boundary position l is -0.5 (l=-0.5). A hatched portion is a part having a low reflection factor, while a remaining white portion is a part having a high reflection factor. In this case, the reflection signal intensity center G2 is positioned to the right of the optical center O. The distance between the reflection signal intensity center G2 and the optical center O is E2.
FIGS. 8(e) and 8(f) also show cases where the reflection ratio boundary positions l are 0 (l=0) and +0.5 (l=+0.5), respectively. The reflection signal intensity centers are G3 and G4, and the distances are E3 and E4, respectively.
In the case FIG. 8(g), the whole of the spot image is covered with a reflecting plane having a low reflection factor. The optical center O coincides with the reflection signal intensity center G5, and the distance E5 is zero. The distances E1-E5 shown in FIGS. 8(c)-8(g) are plotted along the ordinate, and thus obtained points are connected with lines so as to obtain the graph of the distance E in FIG. 8(b).
This distance E is the amount corresponding to the contrast blur, which will be explained with reference to FIGS. 9(a) and 9(b).
FIG. 9(a) corresponds to FIGS. 2(a) and 2(b). When a value (VA-VB)/ VA+VB, which is an AF signal in a case where the light spot image S on the light sensing element 8 of the infrared spot projected onto the object having even reflection factor moves relatively through the positions (I)-(II)-(III) from the area 8A to the area 8B on the light sensing element 8, is plotted, the characteristic curve, shown with a solid line in FIG. 9(b), can be obtained. Namely, when the geometrical center O of the light spot image S comes on the boundary line between the areas 8A and 8B of the light sensing element 8, the AF signal becomes zero and, therefore, the contrast blur does not take place.
When, as shown in FIG. 9(a), the AF signal in a case where the light spot image S, consisting of the contrast pattern with l.apprxeq.0.6 and k=8, relatively moves on the light sensing element 8 through the positions (I)-(II)-(III), is plotted, the AF signal is represented as shown by a broken line in FIG. 9(b), in which the zero-cross position is deviated by .DELTA.x. The light spot image S at the zero-cross position is at the position (II) in FIG. 9(a), while the signal intensity center G is on the boundary line between the areas 8A and 8B of the light sensing element 8. Consequently, the distance E in FIG. 8(a) corresponds to .DELTA.x in FIG. 9(b), and the AF signal becomes zero at the position at which the spot image S is divided by .DELTA.x on the light sensing element 8.
Where the length of the base line for triangulation is "bL", the focal length of the light receiving lens is "f.sub.S ", the focal length of the photographic lens is "f", and the defocus amount on the focal plane is ".DELTA.b", the following condition is obtained: ##EQU1## That is, contrast blur proportional to .DELTA.x takes place.
When the contrast ratio k becomes larger, the maximum value of the distance E in FIG. 8(b) moves upward to the right, and .DELTA.x in FIG. 9(b) becomes larger. Accordingly, the contrast blur becomes larger.